The present invention relates in general to an optical switching system which is capable of distributing and controlling a signal light composed of light pulses in a communication wavelength band with a pulse-like control light. In particular, the invention relates to an optical switching system utilizing characteristics of an absorption change of a light which a carbon nanotube has.
A carbon nanotube (CNT) is a tubular carbon crystal into which a graphene sheet is rounded, and may take any state from metal to semiconductor depending on a rounding state of the graphene sheet. The carbon nanotubes are classified into a single wall carbon nanotube (SWNT) composed of a graphene sheet of a single layer, and a multi-wall carbon nanotube (MWNT) in which graphene sheets are made into a multi-layer structure. These carbon nanotubes are being studied as a material for a fuel cell, a hydrogen occulusion, a field emission source or the like in the various fields due to a unique property thereof (refer to an article of xe2x80x9cThe Foundation of Carbon Nanotubexe2x80x9d, by Yahachi Saitoh and Shunji Bandoh, CORONA PUBLISHING CO., LTD., 1998, or the like). However, the study for carbon nanotubes till now mainly aims at the application to electric and electronic materials, and hence it may not safely be said that they have been sufficiently examined for optical application.
In case of the optical application, an access to a single carbon nanotube with a fine probe as in case of the application to electric and electronic devices is difficult to be made. Hence, an access to an aggregate of a carbon nanotube with a beam condensed so as to have a diameter of several hundreds nm to several tens xcexcm is chiefly carried out. As the chief reasons that the examination for the optical application gets behind the examination for the application to the electric and electronic materials, it is conceivable that it is difficult to obtain highly pure carbon nanotube samples, especially, SWNT samples, in a scale required for the optical evaluation, and any of carbon nanotubes is difficult to be dissolved into solvent so that it is difficult to obtain an optically uniform films, and so forth.
With respect to the optical applications, the SWNT which has a single absorption band, and which is advantageous in comparison of the optical characteristics thereof with calculated values is chiefly being examined. In an example in which non-linear optical constants of the SWNT in a state of being dispersed in solution are evaluated in 1,064 nm, 532 nm and 820 nm as a non-resonance region, such large linearity as to expect the practicability has not yet been reported (refer to an article of X. Liu et al.: Applied Physics Letters, 74(1999), pp. 164 to 166; Z. Shi et al.: Chemical Communications, (2000), pp. 461 to 462).
On the other hand, the calculation result reveals that the SWNT has an absorption band in the range of 1.2 to 1.6 xcexcm as a communication wavelength region depending on a diameter of a tube (refer to an article of H. Kataura et al.: Synth. Met., 103(1999). pp. 2555 to 2558). In addition, it is reported that a tube diameter can be controlled by a temperature in manufacturing the SWNT (refer to an article of O. Jost et al.: Applied Physics Letters, 75(1999), pp. 2217 to 2219). These results suggest the possibility that the resonance effect of the SWNT can be utilized in the communication wavelength region.
We, on the basis of the foregoing notion, made examination with respect to the non-linear optical effect under the condition of the resonance in the communication wavelength region of the SWNT. As a result, we found out that an SWNT thin film shows absorption saturation of high efficiency in that wavelength region, and by utilizing this phenomenon have completed optical elements, each using an SWNT thin film, each of which is operated in the communication wavelength region (refer to Japanese Patent Application No. 2001-320383).
The optical elements thus found out are an optical switch, a saturable absorption mirror, a waveform shaping unit and the like each of which utilizes high efficient absorption saturation in the communication wavelength region of the SWNT. Though these elements provide non-linear optical elements which are respectively low in cost and can be operated at high efficiency, the performance evaluation thereof is carried out only with respect to the static performance such as driving energy, and hence the dynamic performance such as time response characteristics for a control light has not yet been evaluated.
For constructing an optical switching system which can be operated in the communication wavelength region and at a very high speed, it is necessary to evaluate the dynamic performance, more specifically, the time response characteristics for a control light. That is to say, for constructing a very high speed optical switching system using the carbon nanotubes, it is required that a speed of recovery of an absorption change induced by application of the control light is sufficiently high (a time constant of recovery of absorption saturation is very small). Hence, in a stage in which there is found out the above-mentioned optical elements which are not yet evaluated with the dynamic performance thereof, the construction of an optical switching system which can be operated at a very high speed belongs to an unknown province.
The characteristics required for an optical switching system, in addition to high efficiency and high speed response as described above, are excellent processability, high productivity, a low cost, possibility of promotion of an increased area, a high S/N ratio and the like. Hence, there has been desired a material meeting these characteristics at a high grade.
In the light of the foregoing, the present invention has been made in order to solve the above-mentioned problems associated with the prior art, and provides an optical switching system which is capable of being operated at a very high speed in a communication wavelength region and of meeting the various requests at a high grade under a condition in which carbon nanotubes are submitted for the optical applications.
The present inventors made evaluation of the dynamic performance of a thin film made of carbon nanotubes (hereinafter referred to as xe2x80x9ca carbon nanotube thin filmxe2x80x9d for short when applicable), more specifically, the time response characteristics of an absorption change of the carbon nanotube thin film in the communication wavelength region. As a result, we confirmed that the absorption change induced by application of a control light recovers at a very high speed within a period of time of 1 ps (ps=10xe2x88x9212 sec). Then, we have completed an optical switching system which has a novel construction using a thin film made of carbon nanotubes, and which can be operated at a very high speed on the basis of the important knowledge concerning the function of the thin film made of the carbon nanotubes.
That is to say, according to the present invention, there is provided an optical switching system including: a light control portion to be irradiated with a signal light composed of a light pulse train; a control light applying unit that applies a pulse-like control light synchronized with the signal light to the light control portion to selectively transmit a light pulse train within the signal light to form an output signal light; a clock extracting unit that synchronizes the control light applied by the control light applying unit with the signal light; and a signal detecting unit that receives the output signal light, in which the light control portion is composed of a thin film made of carbon nanotubes.
According to the present invention, the light control portion is composed of a thin film made of carbon nanotubes and hence an absorption change induced by application of the control light recovers at a very high speed. Thus, a pulse-like light is used for each of the signal light and the control light, whereby a signal light pulse train having a very high repetitive frequency can be processed by applying the control light synchronous with the signal light pulse train to the light control portion by the control light applying unit. As a result, an optical switching system is realized which can be operated at a very high speed.
In the optical switching system of the present invention, the light control portion is composed of the thin film in which carbon nanotubes which can be formed by application are heaped. Hence, the optical switching system is excellent in processability, high in productivity, and low in cost, and also promotion of an increased areas is possible therefor.
In the optical switching system of the present invention, it is possible to process the above-mentioned signal light, more specifically, the signal light pulse train having a high repetitive frequency on the order of 109 to 1012 pulses/sec.
In the optical switching system of the present invention, the repetitive frequency of the control light can be made equal to or lower than {fraction (1/10)} of the repetitive frequency of the signal light. The optical switching system is constructed so that plural signal light pulses are controlled with one control light pulse, whereby the signal light can be controlled with the control light having a low repetitive frequency. Consequently, the control for a light pulse can be carried out at a very high speed.
In the optical switching system of the present invention, each of the signal light and the control light can be composed of a light pulse with a time width on the order of 10xe2x88x9212 to 10xe2x88x9215 sec.
In the optical switching system of the present invention, the thin film made of carbon nanotubes preferably contains a single wall carbon nanotube in which absorption saturation is induced at high efficiency. The thin film contains a single wall carbon nanotube, whereby an absorption wavelength region can be made to fall within the range of 1.2 to 1.6 xcexcm, and hence the signal light control in the communication wavelength region can be carried out.
In addition, since the absorption wavelength region is satisfactorily set in the above-mentioned range with a carbon nanotube a diameter of which is in the range of 0.8 to 1.3 nm, it is preferable to contain a carbon nanotube having such a range.
The thin film can be formed by spray-applying dispersion liquid having a carbon nanotube dispersed in a dispersion medium. Since the thin film can be thus formed in a simple manner by the application, the optical switching system is excellent in processability, high in productivity, and is low in cost, and more over, the promotion of an increased area is readily carried out therefor.
At this time, as for the dispersion medium, for example, dimethylformamide can be used.
A thickness of the thin film is preferably in the range of 100 to 600 nm.
In the optical switching system of the present invention, a first condensing unit that condenses the signal light to be applied to the light control portion on an irradiation surface of the light control portion is preferably arranged in a path along which the signal light travels. By thus condensing the signal light to be applied to the light control portion to increase an intensity thereof, it is possible to enhance the throughput of the signal light in the light control portion.
In the optical switching system of the present invention, a second condensing unit that condenses the control light to be applied to the light control portion on an irradiation surface of the light control portion is preferably arranged in a path along which the control light travels. For the control light applied to the light control portion, the light having a high intensity to some extent is required for the purpose of inducing the absorption saturation of a light. Then, since the control light to be applied to the light control portion is condensed in such a manner to allow the intensity to be increased, the control light applied from the control light applying unit can be suppressed all the more to allow the energy efficiency to be enhanced.
At this time, a diameter of a spot of the signal light condensed by the first condensing portion and/or the control light condensed by the second condensing portion on the irradiation surface of the light control portion is preferably in the range of 10 to 200 xcexcm.
In the optical switching system of the present invention, it is preferable that the irradiation surface of the light control portion be divided into plural areas, and the individual areas obtained through the division be optically controlled independently and in parallel. If the irradiation surface of the light control portion is divided into plural areas, and the light control is carried out in such a manner, then light pulses of a large quantity of signal light can be controlled at a time on one surface to make a very high speed operation possible. Note that, xe2x80x9cdivision into areasxe2x80x9d in the present invention does not mean the physical division, but means the division into areas for which optical switching functions function independently of one another. Thus, the division concerned becomes seemingly the virtual division. Of course, the physical division may also be available.
In particular, if a construction is adopted in which light pulses of plural signal lights are controlled independently and in parallel for the respective areas which are obtained by dividing the irradiation surface of the light control portion with a light pulse of one control light, then the repetitive frequency of the control light can be lowered (the repetitive frequency of the control light, for example, can be made equal to or smaller than {fraction (1/10)} of that of the signal light) to allow a signal light having a very high repetitive frequency to be controlled.
As for the optical switching system having such a construction, there is given one having a construction including a parallelizing unit that enlarges the signal light to be applied to the light control portion so as cover the entire plural areas obtained through the division and converts the resultant light into parallel signal lights, in which the control light applied from the control light applying unit is applied to the light control portion so as to have such spreading within a surface perpendicular to a travelling direction (and so as to make a predetermined angle with the parallel signal light if necessary) as to cover the entire plural areas obtained through the division.
As described above, according to the optical switching system of the present invention, it is possible to carry out light pick-up and light distribution of a highly repetitive signal light in accordance with the control light.
Note that, very recently, a paper concerned with the light response characteristics of the carbon nanotube in which the phenomenon similar to the high speed response characteristics of the carbon nanotube in the present invention is described was reported and at the same time, U.S. Patent Application thereof was filed with the Patent Office (refer to an article of Y.-C. Chen et al.: Applied Physics Letters, 81(2002), pp. 975 to 977; and U.S. patent application (Ser. No.: 10/074,937) by Y. P. Zhao et al. filed on Feb. 12, 2002).
However, though the technique in the above-mentioned article in which the carbon nanotube thin film is described aims at a very high speed optical switch in 1.55 xcexcm as the communication wavelength region, since the absorption lies in 2.1 xcexcm to be out of the resonance, the non-linear optical performance is limited to about {fraction (1/1,000)} of a maximum value of the carbon nanotube thin film in the present invention. In addition, in the first place, the carbon nanotube thin film described in the above-mentioned article is a film which is formed by dispersing carbon nanotubes in polymer. Thus, an absorption coefficient thereof in 1.55 xcexcm is also so small as to be about {fraction (1/100)} of that of the carbon nanotube thin film in the present invention, and hence the application of the carbon nanotube concerned to a vertical incidence type optical switch as will be shown in the later embodiments is accompanied with difficulty.
From the foregoing, it can be said that the present invention is different in constitution from the carbon nanotube thin film and the optical switch using the same described in the above-mentioned article, and hence can provide a very high speed optical switch having higher efficiency than that of the optical switch described in the article.